Previous studies have indicated that the accumulation of UDP-glucose pyrophosphorylase during differentiation of the cellular slime mold, Dictyostellium discoideum, results from a decrease in the rate of enzyme inactivation coupled with a fairly constant rate of enzyme synthesis. The proposed research is directed towards establishing whether the observed modulation in enzyme level results from a decrease in the rate of enzyme degradation, or from unmasking of a pre-existing, catalytically inactive pool of enzyme protein. Density labeling experiments employing O18 water will be utilized in order to distinguish between these alternatives, and to more accurately define rates of synthesis and turnover of enzyme protein during differentiation. Factors influencing the in vivo and in vitro rates of enzyme inactivation (proteases, masking proteins, and/or proteins catalyzing a possible post-translational modification of enzyme structure) will be characterized.